Wessex and the Danes
From 'A History of the British Nation' by AD Innes, 1912
In 793 and 794 for the first time Danish longships swooped down upon the monastery of Lindisfarne and the monastery of Jarrow to slaughter and plunder. Somewhere about the same time three pirate crews landed in Dorsetshire and slew the reeve of the shire. But forty years passed before their raiding began in earnest.
In the interval a strong man had arisen in Wessex; and Ecgbert had wrested from Mercia the English supremacy which was to remain with his house permanently, or at least with little intermission, until the Norman seized the sceptre. Ecgbert, who claimed kinship with both the royal houses of Wessex and Kent, had only recently returned from exile hi the land of the Franks when the Witan or Council of Wessex called him to the throne. An efficient king, Coenwulf, was ruling in Mercia, and Ecgbert made no attempt to challenge his over-lordship. But when Coenwulf was succeeded in 822 by his brother Ceolwulf anarchy once more began to set in in Mercia, and the crown was usurped by Beornwulf.
Still Ecgbert bided his time, nor was he himself the actual aggressor. It would seem that Beornwulf, who had secured the Mercian kingship, invaded Wessex when Ecgbert was engaged on a campaign in Damnonia. Ecgbert, returning, inflicted upon him an overwhelming defeat at Ellandune in Wiltshire. Ecgbert was prompt to follow up his victory, Kent joyfully hailed him as overlord in place of the alien from Mercia. The king of Essex submitted to him, and on his death Essex was simply absorbed into Wessex.
The same fate befell Sussex. East Anglia recovered the independence which it had lost to the Mercians, killed Beornwulf in battle, and allied itself with Ecgbert; and in 829 Ecgbert appears to have had no difficulty in making himself master of Mercia. The alliance with East Anglia was soon converted into the subordination of that kingdom, and even the Northumbrian king made formal submission to Ecgbert as "Bretwalda," the supreme lord of the whole land - a title applied to various earlier kings, from AEthelbert to Offa.
Thus when the Danes reappeared in 834, having left the land in peace for forty years, Ecgbert was undisputed lord of all England, with probably a firmer grip of his dominion than any of his predecessors in the supremacy, with the possible exception of Offa.